The present invention relates to a screen assembly and, more particularly, to an improvement in translucent or transmissive screen assembly for use in a projection type color television system or as a display panels.
A conventional translucent screen assembly used in a projection type color television system is shown in FIG. 1.
Referring to FIG. 1, showing the principle of the optical system, the projection type color television system has three cathode ray tubes (only one cathode ray tube 4 is shown) for projection light beams of different primary colors which are normally red, green and blue, onto the translucent screen S through respective lens assembly 5. The three different color light beams are mixed with each other on the screen S to form a proper color picture. The picture so formed on the screen S is viewed by the observers who are occupying at the right-hand side of the screen, that is, at one side of the screen opposite to the side of the tubes.
In order to increase the brightness on the screen, generally the screen gain (amount of light projected on the screen) is increased by enlarging the effective aperture size of the lens assembly 5. However, due to various reasons such as difference in angle of incidence of the light beam from the cathode ray tube, the increased screen gain results in such a disadvantage that the picture at the center portion of the screen is brighter than that at the peripheral portion. This undesirable phenomenon is generally called a hot spot phenomenon. Therefore, even if the center portion should gain sufficient brightness, the brightness at the peripheral portion will still be under the required brightness. Therefore, when the observer scans over the screen, he still has such a feeling that the brightness is not sufficient.
From the view point of practical manner, it has been found that the observer feels brighter if the picture on the screen has a uniform brightness rather than one particular portion brighter than the other.
In view of this, there have been proposed various types of screen assemblies which uniform the brightness of the picture on the screen. One conventional screen assembly is shown in FIG. 1 and includes a layer of Fresnel plate member 1 and a layer of diffuser plate member 2 which are placed and bonded one over the other. In use, the screen assembly is positioned with the Fresnel plate member 1 facing the cathode ray tube and with the differ plate member 2 facing the observer. The Fresnel plate member 1 has a plurality of grooves formed on the surface facing the cathode ray tube for forming a Fresnel lens 3 thereon.
However, in the above arrangement of the screen S, the reflection effected on the opposite faces of each of the plate members 1 and 2 particularly on the bonded face causes a loss of light beam from the cathode ray tubes and thus results in reduction of the contrast.
Furthermore, since the plate members 1 and 2 are manufactured separately and are then bonded to each other, the manufacturing cost of the screen assembly has been considerably high.
For simplifying the manufacturing steps, one may contemplate that the screen assembly can be formed by the Fresnel plate member 1 only, with its plain surface opposing the Fresnel lens surface being mat finished, such as shown in FIG. 2, or otherwise, the mat finishing can be effected on the Fresnal lens surface as shown in FIG. 3. However, in the former assembly, the ambient light will be diffused on the mat finished surface thereby reducing the contrast of the picture on the screen and may easily cause a damage to the mat finish since it is disposed on the observer side. Whereas in the latter assembly, it is practically difficult to provide a mat finished layer uniformly over the grooved surface particularly when the grooves has a very small pitch such as 0.5 mm to 1.0 mm.